Well having a Tamron SP 300mm F2.8 (60B) and a Pentax 1.7x AFA is one thing, actually using it is another thing

the combination gives you 510mm F4.8, semi-AF the poor mans 500mm F4.5, the combination gives you bokeh CA, purple fringing and of course regular CA, higher levels then pretty much any of my other lenses but not to hard to get rid of or hide and its certainly not in every image.